Tape reader with input clipping circuit including photosensitive means



Aug. 9, 1966 v. z. sMrrH TAPE l'ADER WITH INPUT CLIPPING CIRCUIT CLUDING PHOTOSE -Flled Dec. 5, 1962 NSITIVE MEANS 2y Sheets-Sheetl 1- YELLOW TAPE PRI/V750 0F CHAD/.E35

TAPE

@HR/f TAPE MOVEMENT 0F TAPE P/57' PH 70 TRANS/.5 Tl? 0/FFEREA/CE /FFERENCE en -67.0,? SWITCHES HMPL /F /ER PHO T0 TRANS/.5 TDR LIGHT VNPUT Aug. 9, 1966 v. z. SMITH TAPE READER WITH INPUT CLIPPING CIRCUIT INCLUDING PHOTOSENSITIVE MEANS 2 She\ets-Sheet 2 Filed Dec. 5, 1962 4 .6 5 y, j 0 f W ILMJ. f//f C ||||||Mnllll fwlllldl| United States Patent C) 3,265,900 TAPE READER WKTH INPUT CLPPING CIRCUI'E` INCLUDING IHTSENSIIIVE MEANS Vernon Z. Smith, Wayne, Pa., assigner, by mesue assignments, to Borg-Warner Corporation, Chicago, Ill., a

corporation of Illinois Filed Dec. Tv, 1962, Ser. No. 242,341 7 Claims. (Ci. Z50- 214) This invention is directed to a system for both detecting a c-hange in a characteristic of an input signal to the system and for discriminating between informationdenoting changes and random signal aberration-s having no information content. In one environment, the invention affords a positive and accurate, while still economical, system for a photoelectric reader wlhich assimilates information from a wide variety of tapes (e.g., printed, punched, chadless) even though the tape is scanned at a very high speed.

In the photoelectric reading art, it is desirable to read or detect information appearing on the different varieties of tape noted above. In certain systems such as that disclosed and claimed in Patent No. 3,124,675 of Herman Epstein entitled Photoelectrie Tape Reader, issued March 10, 1964, and assigned to the assignee of this invention, such detection has been accomplished by utilizing a specific light rdefining means, or light pipe, to channel the light from a specific location adjacent the tape to a detection circuit. In such system, a scanning disc is interposed between the light pipes and the detection means, and spaced-apart transparent bands are provided on the disc and separated by opaque areas. Accordingly variations in light level translated through the pipes provide a llight signal which, in effect, is utilized to modulate a second or carrier signal of a frequency which is a joint function of the rotational speed of the disc and t-he spacing between .the transparent bands on the disc. While this system is an accurate and fastoperating means for removing the information from different tapes, it is still desirable to improve system operation and to optimize sys-tem production and costs by, if possible, producing a reader which can operate effectively without light pipes and/or a scanning disc.

It is therefore a primary object of the present invention to provide a photoelectric reader which is simple and economical' to fabricate, and use-s components of average cost and quality.

It i-s another object of the invention to provide such a reader without the necessity of incorporating individual light defining means or pipes for each channel on the tape, and without rapidly moving parts such as a chopper disc.

It is a more specific object of the invention to provide such a reader which not only detects` any change in the level of the input signal, but also accurately discriminates between information-denoting variations in Athe input signal and random aberrations of the signal which do not have any information content.

The foregoing and other objects of the invention are realized, in one embodiment, by providing a system in which at least one characteristic of the inpu-t signal var-ies from a reference value to a second Ivalue to denote a change in the information content. However, certain input signals have a random or noise-type aberration in the reference value of an amplitude at least equal to the eX-tent of the variation between the reference value and 4the second value. Accordingly the system is provided with a first circuit means for determining whether a change in the input signal is only an aberration in the reference value or is `a variation from the reference value to the second value. If the change is an informationdenoting variation, a control signalA is 3,2555@ Patented August 9, i966 passed to a second circuit means, which in turn examine-s the control signal and produces an output signal only when the amplitude of the control signal indicates that the change in the input signal was in fact an information-denoting variation.

In order to acquaint those skilled in the a-rt with the best mode contemplated for making and using the invention, a :description thereof will be set forth in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIGURE l is a graphical illustrati-on useful in understanding operation of the invention;

FIGURE 2 'is a block diagram depicting certain major components of the invention in simplified form;

FIGURE 3 is a schematic diagram illustrating in more detail the inventive structure depicted generally in FIG- URE 2;

FIGURE 4 is a schematic diagram of a power supply arrangement utilized to supply a plurality of circuit arrangements such as that shown in FIGURE 3; and

FIGURE 5 is a partial schematic diagram useful in explaining the operation of a portion of the circuitry depicted in FIGURE 3.

INTRODUCTION FIGURE 1 depicts not only certain of the oper-ating characteristics of the invention, but further illustrates characteristics of the light level reflected from different tapes which are useful in understanding the problems of this art which are so eciently, economically and accurately solved by this invention. The -abscissa depicts movement of tape past the phototransistor or scanning element, and the ordinate indicates the variation in current or signal output from the semiconductor as a hole, blackened area, or other information-denoting portion of the tape is displaced past the scanning point to cause a reduction in the amount of light striking the phototransistor. The term hole as used in FIGURE 1 and in this explanation is considered generic not only to a definite aperture in the tape but also to a blackened area and to a chad or punched area which passes beneath or adjacent the scanning point.

Reference numeral lib designates the curve which represents the signal level detected with a yellow tape, and under certain operating conditions the no-hole signal level may be of the order of 50 microarnperes (microamps), varying from approximately 40 to 60 microamps, and this signal level may decrease to only about one-half microamp as the hole passes beneath the semiconductor scanning means. Thus the reference level of this signal is about 50 microamps, .and this value changes to a second value of 0.5 microamp as a variation in the information content on the tape is detected. Even when no information change is detected, there is an undesired signal aberration of about 20 microamps, `as represented by interval 9 in the drawing.

The broken-line lCurve 11 designates the variation in signal level when printed or chadless tape is utilized, and it h-as been found that the signal variation in this case may approximate a change from about 40 microamps with solid paper to a level of about 10 microamps as a hole passes beneath the scanning point. The curve designated 12 represents the signal variation when a black or dark background punched tape is utilized, and the signal level here may be approximately 4 microamps with solid paper and may decrease to `about 0.2 microamp as a hole passes the scanning point. Accordingly it is evident that a mere detection of the change in the ratio of conduction between the no-hole and the hole conditions cannot be utilized to indicate information changes, in that the signal aberration of the yellow tape reference Value is substantial, being considerably greater than the variation noted with the dark tape when a hole passes the scanning point. Further, a simple D.C. clipping system cannot be utilized by itself, in that the signal level from the dark tape, even under no-hole conditions, is considerably less than the lowest signal value realized from the printed and chadless tapes when a hole passes the scanning point. These conliicting operating conditions appear contradictory and not readily admissible of a straightforward circuit solution.

In accordance with the inventive concept, a light-sensitive or radiation sensitive semiconductor means is positioned to provide an electrical signal responsive to the incidence of energy in the visible portion of the spectrum on the semiconductor. Those skilled in the art will appreciate the efficacy of the invention in discriminating between unwanted aberrations or noise and informationdenoting signal variations, irrespective of the origin of the signal. A first circuit means is `coupled to, and cooperates with, the semiconductor means to establish a clipping level approximately at the amplitude referencediby numeral 13 in FIGURE l. The clipping level is set so that the worst hole signal is accommodated, that is, the signal variation which decreases to a second value which is highest on the ordinate scale (in the illustrated 'case the printed or chadless tape provides a maximum signal when a hole passes the scanning point) is below clipping level 13 by an extent indicated by numeral 14. The difference in amplitudes designated by numeral 14 is important in its relation to the amplitude difference referenced by numeral 15, which identities the signal Variation between the hole and the no-hole conditions for dark tape. The dark tape signal vari-ation represents the worst difference condition, in that the least absolute change in signal levels is realized to signify the difference between the hole and no-hole conditions, In accordance with the inventive teaching, by (l) providing difference detection for a signal change at least equal to the extent represented by numeral 15, and (2) setting the D.C. clipping level 13 so that the interval 14 is always equal to or greater than the interval represented by numeral 15, positive and accurate signal detection is obtained to represent the passage of holes or darkened areas past the scanning point.

FIGURE 2 shows, in simplified form, a system for attaining the objects of the invention. A phototransistor 16 is provided in order to produce a useable variation in signal output as the level of light incident on the phototransistor changes to signal passage of la hole. Although sometimes characterized as a photodiode, the semiconductor means utilized in one embodiment (specific examples will be set out hereinafter) was actually a threeelement semiconductor, even though only two electrical connections are provided. Essentially the base region has the generation of minority carriers regulated by the level of light incident thereon, in a manner analogous to the regulation of the carriers by the application of a Voltage to the base in a conventional three-terminal transistor. A D.-C. clipping stage 17 is coupled to the phototransistor. As will become apparent from the explanation given hereinafter, D.-C. clipping stage 17 interacts with the phototransistor to provide clipping at a level such as that indicated by numeral 13 in FIGURE l, providing a control signal output whenever the input signal falls below the level referenced by numeral 13.

A difference 'amplifier is coupled to the output side of D.C. clipping stage 17, and a difference detector stage 19 is coupled to the output side of the difference ampliiier. The difference detector is adjusted to provide an output signal responsive to the application to its input side of a control signal having at least an amplitude such as that referenced by interval in FIGURE 1. Although not essential for the operation of this novel system, one or more switches 2@ are `coupled between difference detector 19 and output terminal 21 to enhance the level of the output signal and provide for positive operation of associated equipment.

Before considering the structure of the invention in more detail, it is again emphasized that the invention is concerned not only with structural improvements but also with optimizing orf cost factors and the production of a reliable circuit which will operate even when average-cost germanium transistors are utilized. Further, the ever present dilemma of choosing between gain and stability in the overall circuit, two characteristics which normally are mutually opposing, is resolved by the unobvious system olf the invention which attains maximum practical levels of both gain and stability without sacriiicing the economy or sureness of operation.

STRUCTURE OF THE INVENTION In FIGURE 3, D.C. clipping stage 17 is shown coupled between a first supply terminal 22 and a second supply terminal 26, to which sui-table energizing potentials are applied as will [be described hereinafter. Coupled in series between terminals 22 and 23 are a iirst resistance 24, a second resistance 25, semiconductor 16, a third resistor 26, and a fourth resistor 27. Difference amplifier 118 comprises la pair of transistors 2S and 29, each of which includes an emitter identified by e (e.g., 28e identifies the emitter of transistor 28), a ibase element referenced b and a collector designated c. Those skilled in the art will recognize that although 'PNP type transistors are shown, other switching means such as NPN transistors (or even electron-discharge devices) can Abe substituted therefor with the appropriate changes in the polarities of the operating and energizing potentials applied to such units.

A iirst coupling capacitor 30 is intercoupled 'between base 2817 of transistor 28 and the junction orf resistors 24 and 25. Another coupiing capacitor 311 is coupled between base 29h and the junction of resistors 26 and 27 in the D.-C.- clipping arrangement. A first emitter resistor 32 has one end thereof coupled to emitter or cornmon electrode 23e, and `a second emitter resistor 33 has one end thereof connected to emitter 29e. The other ends olf these two resistors lare coupled together, and this common connection is coupled over a dropping resistor 64 to line 315, in its turn `connected to a terminal 36 to which a -suitalble unidirectional operating potential is applied. A biasing resistor 37 is coupled between base or input electrode 28h and common or ground line 38, and another biasing resistor 39 is coupled between base 29b and common line 38. Collector or output electrode 28C is coupled over a dropping resistor 40 to a supply line 41, in its turn coupled to terminal 42 to which a suitable unidirectional operating potential is applied. Another collector dropping resistor 43 is coupled between collector 29e and conductor 41.

Difference detector 19 includes a transistor 44, which in this embodiment is depicted as a PNP type transistor, having an input electrode or btase 44h, an output electrode or emitter 44e, and a common electrode or collector 44C. A coupling capacitor 45 is intercofupled between base 44h Iand collector 29C orf transistor 29. A diode 46 is interconnected as indicated between common line 38 and base Mb. Resistor 47 is intercoupled between collector 44C and supply line 41. Emitter 44e is coupled over resistor 4S and another resistor 419 to conductor 325. An output path from this emitter-[follower stage 44 is provided from the junction of resistors 48 and 49 to base 50h of transistor 50 in the switching circuitry.

Emitter 50e is coupled to ground line 38, and collector 50c is coupled over a resistor 51 to supply line 411; this collector is also coupled over another resistor 52 to base 53h of another switching transistor 53. Collector 53C is coupled 'both to output terminal 21 and over a resistor 154 to supply line 41. Emitter 53e is coupled directly to ground line 68, and base 53b is coupled over a resistor 515 to line 35.

When a plurality of different channels are read or scanne-d with a reader utilizing the circuitry of the invention, an individual amplifier such as that shown in -the desired sensing semiconductor energizing potential.

A filter capacitor 6'3 is connected across resistor 61 to assist in stabilizing the voltage across terminals 22 and 23. The other terminals designated 42, 36 and 56 Iare connected as indicated 'by like reference characters in FIG- URE 3.

OPERATION OF TH-E INVENTION As tape with unvarying brightness or unpunched areas is displaced past sensing phototransistor 16, the level orf light incident on the phototransistor remains substantially unchanged, so that the conductivity of semiconductor 1 6 is substantially constant and the current ow through the circuit including resistors 24-27 and semiconductor 16 is substantially unchanged. Likewise there is constant conduction of transistors 2'8 and 29 in the difference ampliiier, so that the potential appearing at collector 29e and applied to one side off capacitor 45 remains unchanged.

As a hole or blackened area is displaced past sensing semiconductor 16, the level of radiation sensed thereat is reduced, as indicated in FIGURE l, decreasing the conductivity of the semiconductor and thus decreasing the current ow between terminals 22 and 23. The laction of this circuit in producing the desired signal output over capacitors 30 and 31 to difference ampliiier 18 will be explained in connection with FIGURE 5.

As there indicated, the phototransistor or semiconducttor sensing means is depicted by a current generator 70 and an impedance 71, depicted as a resistor. One side of this parallel combination of elements is coupled through resistors 25 .and 24 to a point of reference potential, designated 72; the polarity of this voltage is negative with respect to ground. Thelother common terminal of the parallel combination of current generator 70 and impedance 71 is coupled over resistors 26 and 27 to a terminal 73, to which a positive unidirectional energizing potential is applied. Between these terminals a pair of capacitors 74 and 75 are coupled in series, with 'the common connection between the capacitors coupled to ground lat 76. This real ground connection produces a virtual ground 7 '7 at a symmetrical position in the semiconductor sensing arrangement, provided that 'the circuit elements are sized properly.

A certain signal voltage will appear 4on each of the output conductors 78 and 79. Assuming initially that the level of light incident on the phototransistor is high, a high current is generated in unit 70 and is translated through each of resistors 24-27. With the current generator operating at a high level, at some point the IR drop across resistor 25 (which appears :at output conductor 78) will reach a maximum value, being limited by the value of the supply voltage applied to point 72. Accordingly it is not until the level of current drops to such a value that `the IR drop .across resistor 25 decreases that an output signal variation appears on conductor 7S.

As the current through .resistors 25 and 24 decreases, the voltage d-rop across resistor 25 likewise decreases so that the potential at the junction of resistors 24 ,and 25 changes and approaches that |appearing at terminal 72, that is, the potential at this junction becomes more negative. Such circuit operation is referenced in FIGURE 3 by the indication of the negative polarity sign to the left of capacitor 30. An analogous operation takes place in the lower portion of the circuit shown in FIGURE 5, resulting in the appearance of a voltage with the polard ities indicated across the capacitor 31 in FIGURE 3. Accordingly, the negative-going voltage appearing between resistors 24 and 25 is applied through capacitor 3i) to base 2gb of transistor 28, concomitantly with the application jof la positive-going voltage through capacitor 31 to base 29h tof transistor 29.

Each of transistors 28 and 29 is, in the illustrated embodiment, of the PNP type. Accordingly the application of the negative-,going voltage `to base 28h effects an increase in the collector current of transistor 28, and the positive-going voltage .applied to base 2911 causes Ia rapid decrease in the collector current to transistor 29. With this sharp decrease in the collector current flowing lthrough resistor 43 to supply line 41, the voltage across resistor 43 is rapidly reduced, with the voltage at the bottom of 4resistor 43 swiftly approaching the potential at terminal 42. In consequence, a negative-going pulse such as that referenced by numeral is provided at the bottom of this resistor. As soon as the hole passes the A.scanning point, an opposite potential change occurs across capacitors 36 land 31, and normal circui-t operation is restored to provide the trailing edge of pulse 80. This negative pulse is applied through coupling capacitor 45 to `the common point between base 44b and diode 46 in the dilference detector circuit.

Transistor 44 in diiierence detector circuit 19 is connected in an emitter-tiollower configuration. In this circuit, it is the potential appearing at the junction of resistors 4S and 49 which is Iapplied to lthe base Sb of the first switching transistor 50, and such potential therefore determines the conductivity of that stage. When a negative-going pulse of suiiicient amplitude is applied to PNP type transistor 50, the transistor will be gated on. The value of impedance between base 50h and the positive supply terminal 36 is set, being equal in the illustrated embodiment to the value lof resistor 49. In the other direction from base Stb, three different components make up the total impedance value between `the base and ground. These components include resistor 48, the emitter-base impedance of transistor 44, .and the impedance exhibited by diode 46. These parameters are chosen so that to effect a sufficient change in the circuit values to cause transistor 50 tto conduct, pulse 80 must signify a variation of input signal amplitude at least equal to that indicated by interval 1S in FIGURE 1. It is this cornbination of dileren-ce detection and effective D.C. clipping in the initial portion of the system that contributes substantially to the success of the invention.

It is noted that diode 46 also provides for D.C. clamping after the signal has passed through coupling capacitor 45, land that the emitter-follower coniiguration of transistor 44 contributes to a power gain of the signal. In the absence of receipt of any hole `or dark tape signals, transistor 50 is non-conducting and transistor 53 is conducting. Accordingly when a control signal 8i) representing 1a variation greater than the interval 1S in FIGURE l is applied over capacitor 45 to this difference detection circuit, the negative pulse applied to base 50h gates this transistor on and increases the iiow of collector current through resistor 51 to supply line 41. Thus the potential appearing at the junction ot resistors 51 and 52 changes in a positive direction, and a more positive voltage is applied to base 53b to switch this transistor oil, providing -a negative-going signal at output terminal 21. Those skilled in the art will recognize that a useable signal is provided at the output side of the diiference detector (eg, the junction 'of resistors 4S and 49), and that Ethe use of switching transistors 56 and 53 does not iaiiect the inventive concept and structure.

scanner, or other signal discrimination circuitry. In particular, the separate considerations of temperature stability, D.-C. stability and good gain characteristics have all been realized in the structure disclosed and claimed hereinafter. These improved operating characteristics have been realized with inexpensive components, such as germanium transistors rather than the more expensive silicon types. With the accurate and economical circuitry here disclosed, it is possible to provide an efficient photoelectric reader without individual light channels or light pipes and without the rotating disc.

To facilitate the construction and use of the invention, a table of suitable component identifications and values of the various elements shown in FIGURES 3 and 4 is set out below. It is emphasized, however, that such a table is given by way of illustration only and in no sense by way of limitation of the inventive scop-e.

Component Identification Phototransistor 16 1N2l75 Transistor 28 2N508 Transistor 29 2N508 Diode 46 1N279 Transistor 44 2N508 Transistor 50 2N404 Transistor 53 2N404 Resistors: Value in ohms 24 100K 25 270K 26 270K 27 100K 32 470 33 470 34 22K 37 33K 39 33K 40 22K 43 22K 47 1.5K 48 47 49 2.2K 51 2.2K 52 4.3K 54 1K 55 56K 60 2.2K 61 4.7K 62 2.2K

Capacitors: Values in microfarads 30 1 (25 v. D.C.) 31 l (25 v. D.C.) 45 5 (25 v. DC.) 63 50 (50 v. D.C.)

In addition, a negative l2 volts energizing potential was 'appli-ed to terminal 57 in FIGURE 4, land a positive twelve volts potential was applied to terminal 58, each being referenced with respect to the common or ground terminal 59.

It is noted that resistors 25 and 26 were selected to be substantially larger in ohmic value (more than twice as large) than resistors 24 and 2'7. Such sizing contributes to the isolation and noise immunity of the invention, especially to environmental noise such as is generated by the drive motor of the system.

While only a particular embodiment of the invention has `been described and illustrated, it is apparent that modifications and alterations may be made therein. It is therefore the intention in the appended claims to cover all such modifications and alterations as may fall Within the true spirit and scope of the invention.

What is claimed is:

l. In a signal examining and indicating system for receiving input signals varying from a reference value to a second value to indicate information content, said input signals 'being subject to undesired aberrations and also to environmental noise occurring above a given amplitude level, the improvement which comprises clipping circuit means including a photosensitive semiconductor means, rst impedance means coupled between said semiconductor means and a first plane of reference potential, second impendance means coupled between said semiconductor means and a second plane of reference potential, -current iiow through the series circuit including said semiconductor means and said rst and second impedance means varying as a function of the level of the radiation incident on said semiconductor means, circuit means for obtaining a control signal from said series circuit only responsive to a variation in the radiation level indicating receipt of an input signal varying below said given amplitude level, and a difference detector circuit coupled to said circuit means to provide -an output signal only responsive to a determination that the control signal in fact indicates a variation between said reference and said second values of the input signal.

2. In a signal examining and indicating system for receiving input signals varying between a reference level and la second level to denote information content, certain of said input signals being subject to an undesired aberration in the reference level signal which occurs above a given amplitude level and said system further being subject to environmental and other noise also occurring above said given amplitude level, the improvement which cornprises a clipping circuit comprising a series circuit coupled between two different planes of supply potential, said series circuit comprising a first impedance, a second impedance, a photosensitive semiconductor unit, a third impedance, and a fourth impedance, said clipping circuit passing `a control signal responsive to a variation of said input signal -below said given amplitude level, a difference amplifier including at least two input connections and one output connection, means for coupling the junction of said rst and second impedances to one input connection of said difference amplifier, means .for coupling the junction of said third and fourth impedances to the other of said input connections of said difference amplifier, a difference detector circuit for receiving the control signal from the difference amplifier whenever the variation of input signal indicates receipt of an input signal with a variation of information content, 4and means for coupling said output connection of the difference amplifier with said difference detector circuit to provide a positive indication of receipt of information-denoting signals in said I system.

3. A system as set out in claim 2 in which said second and third impedances are substantially larger in value than said first `and fourth impedances, thereby to improve the immunity to noise of the clipping circuit.

4. In a signal indicating system for receiving input signals varying 'between reference and second levels to depict information content, said system being subject to random noise and to aberrations in the reference level of different information-denoting signals, all of said random noise and level aberrations occurring above a given amplitude level of the input signals, the improvement which comprises a clipping circuit for receiving the input signals, said clipping circuit comprising photosensitive semiconductor means and being adjustable to operate in a saturated condition at said given amplitude level Whenever signals of said reference level, undesired aberrations at approximately said reference level, and unwanted noise are received by the system andto operate at a second amplitude lower than said given amplitude level only responsive to an amplitude variation in an input signal below said given amplitude level, and a difference detector circuit coupled to said clipping circuit for providing an output signal only responsive to receipt of -a control signal representing a decrease in the amplitude of the signal from said clipping circuit by a preassigned amount, said difference detector means including semiconductor means in the circuit which determines conductivity of this circuit, which semiconductor means changes impedance as Aphotosensitive 9 the control signal is received to insure fast action of the difference detector circuit and a positive indication of the output signal.

5. In a system for both rejecting noise and unwanted signal Iaberrations which occur above la given amplitude level and for passing information-denoting signals which vary from a reference level to a second level lbelow said given amplitude level, a clipping circuit calibrated to provide no signal output Whenever the level of the input signal is above said given amplitude level and to provide a control signal output whenever the amplitude of said input signal varies below said given amplitude level, and a difference detector circuit coupled to said clipping circuit, said dierence detector circuit comprising an input connection coupled to said clipping circuit and an output connection, a rst semiconductor having input, output and common electrodes, a second semiconductor, means for coupling said input ,connection both to the input electrode zof said first semiconductor means and to said second semiconductor, and means for intercoupling said output electrode with the output connection of the difference detector circuit, wherebyapplication of a control signal from said clipping circuit to the input electrode of said first semiconductor eifects rapid switching of the difference detector circuit from a irst state to a second state to provide an output signal indicating a change of information in said input signal.

6. In a photoelectric reading arrangement `for removing information from a plurality of different types of tapes having diferent background levels, the improvement which comprises an input clipping circuit comprising semiconductor means and impedance means intercoupled to establish a clipping level such that no control signal is provided whenever the input signal to the clipping circuit is above a given amplitude level and to provide a control signal only when an informationdenoting mark on the tape passes the scanning point and reduces the amplitude of the input signal to the clipping circuit below said given amplitude level, a difference Iamplifier coupled to said clipping circuit for enhancing the amplitude of the control signal, a difference detector circuit coupled to said difference amplier for providing an output signal only responsive to the receipt of a control signal of preassigned -amplitude from said dilerence amplier, and switching means coupled to said difference detector circuit to amplify the output signal lfor translation to associated equipment.

'7. In a photoeleet'ric tape reader for transferring information from any of a plurality of different types of tape, each different type of tape being related to a given average electrical signal when no hole appears on the tape, said signal decreasing to a hole-denoting minimum value as a hole on the tape moves past the scanning location, an input clipping circuit comprising photosens itive semiconductor means and a plurality of impedance means coupled to the semiconductor means, said clipping circuit being operative at a clipping level above the maximum level of the various hole-denoting minimum values for the different types of tape so that the worst hole signal is accommodated and a control signal is provided by the clipping cir-cuit when a hole on the tape passes the scanning looation, a difference amplifier coupled to the clipping circuit for increasing the amplitude of such control signal, a difference detector circuit coup-led to the difference ampliiier for accommodating the worst difference signal Iby providing an output signal only when the amplified control signal exceeds as preassigned amplitude, and means coupled to the difference detector circuit to pass the output signal to associated equipment.

References Cited bythe Examiner UNITED STATES PATENTS 2,507,743 5/1950 Victoreen Z50-219 2,657,258 10/1953y Hestor Z50-206 X 2,825,818 3/1958 Richardson Z50-214 X 2,941,083 6/ 1960 Henderson et al 250--206 3,001,077l 9/1961` Van Overbeek et al. Z50-214 X 3,058,004 10/1962 Domizi et al. Z50-219 3,061,731 10/1962 Thier et al. 250219 3,135,867 6/1964 Dane 250-214 X 3,163,746 12/1964 Hoeser Z50-219 FOREIGN PATENTS 230,965 10/ 1960 Australia.

RALPH G. NILSON, Primary Examiner'.

E. STRICKLAND, M. A. LEAVITT,

Assistant Examiners. Y 

2. IN A SIGNAL EXAMINING AND INDICATING SYSTEM FOR RECEIVING INPUT SIGNALS VARYING BETWEEN A REFERENCE LEVEL AND A SECOND LEVEL TO DENOTE INFORMATION CONTENT, CERTAIN OF SAID INPUT SIGNALS BEING SUBJECT TO AN UNDESIRED ABERRATION IN THE REFERENCE LEVEL SIGNAL WHICH OCCURS ABOVE A GIVEN AMPLITUDE LEVEL AND SAID SYSTEM FURTHER BEING SUBJECT TO ENVIRONMENTAL AND OTHER NOISE ALSO OCCURRING ABOVE SAID GIVEN AMPLITUDE LEVEL, THE IMPROVEMENT WHICH COMPRISES A CLIPPING CIRCUIT COMPRISING A SERIES CIRCUIT COUPLED BETWEEN TWO DIFFERENT PLANES OF SUPPLY POTENTIAL, SAID SERIES CIRCUIT COMPRISING A FIRST IMPEDANCE, A SECOND IMPEDANCE, A PHOTOSENSITIVE SEMICONDUCTOR UNIT, A THIRD IMPEDANCE, AND A FOURTH IMPEDANCE, SAID CLIPPING CIRCUIT PASSING A CONTROL SIGNAL RESPONSIVE TO A VARIATION OF SAID INPUT SIGNAL BELOW SAID GIVEN AMPLITUDE LEVEL, A DIFFERENCE AMPLIFIER INCLUDING AT LEAST TWO INPUT CONNECTIONS AND ONE OUTPUT CONNECTION, MEANS FOR COUPLING THE JUNCTION OF SAID FIRST AND SECOND IMPEDANCES TO ONE INPUT CONNECTION OF SAID DIFFERENCE AMPLIFIER, MEANS FOR COUPLING THE JUNCTION OF SAID THIRD AND FOURTH IMPEDANCES TO THE OTHER OF SAID INPUT CONNECTIONS OF SAID DIFFERENCE AMPLIFIER, A DIFFERENCE DETECTOR CIRCUIT FOR RECEIVING THE CONTROL SIGNAL FROM THE DIFFERENCE AMPLIFIER WHENEVER THE VARIATION OF INPUT SIGNAL INDICATES RECEIPT OF AN INPUT SIGNAL WITH A VARIATION OF INFORMATION CONTENT, AND MEANS FEOR COUPLING SAID OUTPUT CONNECTION OF THE DIFFERENCE AMPLIFIER WITH SAID DIFFERENCE DETECTOR CIRCUIT TO PROVIDE A POSITIVE INDICATIONOF RECEIPT OF INFORMATION-DENOTING SIGNALS IN SAID SYSTEM. 